1. Field of the Invention
The present invention is directed to the field of acoustic transducers including planar magnetic acoustic transducers and, more particularly, to the distribution of driving forces on the diaphragm of magnetic acoustic transducers, with respect to the edges of the diaphragm which are fixed to a support frame.
2. History of the Related Art
Magnetic acoustic transducers and particularly planar magnetic loudspeakers are generally popular because of their good sound reproduction characteristics. Such loudspeakers typically include a generally flat diaphragm having a pattern of one or more conductors attached which form the "voice coil" or signal current carrying conductors. The diaphragm is positioned so that the conductors are attracted and repelled by adjacent magnets as current signals pass through the conductors, thereby causing the diaphragm to oscillate and produce sound.
The sound reproduction of a typical planar magnetic transducer is sensitive to the operating characteristics of the diaphragm. A typical diaphragm includes a thin flat polymer membrane with a pattern of thin foil-like conductors on the membrane. The conductor circuit, as described and referenced throughout this application, is the pattern of one or more conductors and equivalent terms are conductor voice coil or conductor pattern. Generally elongate portions of the conductor circuit are referred to as conductor runs and equivalent terms are conductor segments or strips. To obtain optimum acoustic response, the diaphragm is held under tension. The path for the electrical conductor runs on the diaphragm is generally chosen so the current flowing through the conductor induces net forces of uniform direction perpendicular to the diaphragm surface during operation of the transducer. Typically, the conductor runs have covered substantially most of the diaphragm so that the "active area" of the diaphragm was the area of the diaphragm not bound at the frame edges, occasionally referred to as the "open area". The generated forces in all of the conductor segments or runs within what is referenced as an "active area" of the diaphragm, cause the general direction of diaphragm motion to be perpendicular to the diaphragm surface.
The sound reproduction characteristics of a planar magnetic transducer are influenced by the shape of the frame, mechanical properties of the diaphragm and conductor pattern, location of the driven area and acoustic impedance from the support frame geometry. Typically the frame shape is rectangular and of such dimensions to produce a desired low frequency resonance as well as a characteristic dispersion at higher frequencies. The mechanical properties of the diaphragm including mass, stiffness, tension and damping all influence the modal behavior and hence frequency response of the transducer. At higher frequencies, the acoustic impedance of the underlying frame will modify the resonant behavior.
Planar magnetic transducers with partially driven areas have been known having magnet circuits and conductor patterns in either a line driver or an array of parallel bars. FIG. 1 shows an acoustic diaphragm 1 and frame 2 having a line driver 3 symmetrically placed in the middle of the rectangular shaped diaphragm as taught in U.S. Pat. No. 4,924,504 to Burton. Passive mass 4 is added to the rectangular diaphragm to control undesired resonant modes. The extra mass has the effect of reducing the output sensitivity.
FIG. 2 shows a three bar array of magnets 5 symmetrically placed in the center of a rectangular frame 6 as taught in U.S. Pat. No. 4,156,801 to Whelan et al. The acoustic transducer includes a diaphragm (not shown) with substantial non-driven area and baffles are provided contacting one side of the diaphragm to control undesired resonant modes of the diaphragm. Such baffles reduced the output sensitivity. An EMIM speaker product sold by Infinity has a design similar to the U.S. Patent to Whelan et al. and uses a combination of damping and stiffening of the diaphragm to control the undesired resonant modes.
FIG. 3 shows another three bar array of magnets 8 symmetrically placed in a rectangular frame 9 with a substantial non-driven area for the diaphragm (not shown) of an acoustic transducer as taught in UK Patent 1545517 to Millward. Again some form of damping such as foam contacting the diaphragm was used to control undesired resonant modes. Such dampening, however, reduces the output sensitivity.
FIG. 4 shows an acoustic transducer 10 of U.S. Pat. No. 3,873,784 to Doschek having a rectangular magnet pattern 11 and conductor layout 12 on diaphragms 14. The magnets and circuits are parallel to a support frame 15 and have reflection symmetry with the axis 16--16 of the frame.
The magnet structures and driven conductor lengths are parallel to the edges of a rectangular frame for these known examples of prior transducers with partially driven area. In addition, the transducers have reflection symmetry about both central axes of the frame. U.S. Pat. No. 3,674,946 to Winey describes a transducer with a triangular frame shape that functions to minimize transverse resonant waves by varying a transverse distance between the frame edges, however in this case the transducer diaphragm is fully driven over the open area, and conductor driving forces are parallel to one edge of the triangular frame.